In the sinking of a shaft into the ground, e.g. for communication with or as a precursor of a tunnel, it is a common practice to preform the upper portion of the excavation and then introduce a shaft-sinking machine into this excavation to increase the depth thereof. Such an apparatus can have a platform which carries the working tools which can include milling cutters or the like for removing material from the floor or face of the shaft and thereby forming detritus which is carried out of the advancing shaft.
It is advantageous with such systems to provide a pilot bore at the center of the shaft to be sunk as an aid to the shaft-sinking operation.
The term "detritus" as used herein is intended to refer to all types of excavation debris, independent of its hardness, and ranging from large hard rock to soft clay materials.
Conventional apparatus of the aforedescribed type is intended to provide a central pilot bore and then to mill away the floor or face of the shaft of the entire cross section or diameter thereof with the detritus at least partly falling into the pilot bore.
Difficulties have been encountered because of the filling of the pilot bore with detritus, in further advancing or sinking the shaft and hence efforts have been made to overcome this disadvantage by slurrying the detritus in the pilot bore (hydraulic conveying) or the like.
Pneumatic conveying of the detritus has proved to be impractical in many cases because of the high energy demand and the inability to pneumatically convey heavy materials over long distances at reasonable costs. Hydraulic techniques have proved to be impractical in many applications, particularly where the shaft is to be sunk in a frozen substructure as is required in instances in which the shaft wall as it is advanced would not otherwise be self-supporting.